Furnace control system



' June 13, 1944. D. E. GARR ET-AL 2,351,420

FURNACE CONTROL SYSTEM Filed March 27, 1943 Inventors: Donald E. Garr, George E. Shaad,

b W 6, KdA/fl flf y Thelr Attorney.

Patented June 13, 1944 FURNACE CONTROL SYSTEM Donald E. Garr and George E. Shaad, Schenectady, N. Y., assignors to General Electric Company, a corporation of New York Application March 27, 1943, Serial No. 480,824

6 Claims.

Our invention relates to furnace control systems, more particularly to control systems for electric electrode furnaces, and has for its object a sensitive and dependable control system which may be operated either as a fully automatic or as a manually controlled system.

In carrying out our invention in one form we raise and lower the electrode by means of a motor which is supplied with current from a generator whose voltage may be varied with precision and great amplification in response to furnace conditions thereby to control automatically the raising and lowering of the electrode during the operation of the furnace.

More particularly, we control the generator by controlling its main field winding in accordance with the difference between two voltages, one of which is responsive to the current in the electrode and the other of which is the voltage across the are between the electrode and the furnace charge. Any difference in these voltages is impressed on the generator field winding thereby to give a generator voltage whose value is dependent upon the value of this difference in voltage, and whose direction is dependent upon the direction of this differential voltage. The motor raising and lowering the electrode thus operates at a speed dependent upon the generator voltage and in a direction dependent upon the direction of the generator voltage thereby to raise or lower the electrode and make the necessary adjust ment to maintain a uniform current in the electrode.

More specifically, we provide a generator for the electrode motor of the armature reaction type whose voltage responds very quickly and with very high amplification to any change in its field excitation, preferably of the type described and claimed in Patent No. 2,227,992, issued to Ernst F. W. Alexanderson and Martin A. Edwards on January 7, 194 1.

We also provide two manually operated control switches for selecting between manual and automatic or partially automatic operation, one switch being a master switch and selecting between manual or automatic operation for all of the electrodes, such as for all three electrodes in a three-phase furnace, and a manual switch being provided for each electrode for controlling separately the generator for that electrode thereby to raise or lower the electrode manually.

For a more complete understanding of our invention, reference should be had to the accompanying drawing in which the single figure is a diagrammatic View of an arc furnace control system embodying our invention.

Referring to the drawing, we have shown our invention in one form as applied to the raising and lowering ofan electrode in an arc furnace S with relation to a'metal charge ll contained in a suitable crucible l2. The electrode l0 and also the other two electrodes of the three-phase furnace shown are raised and lowered each by a suitable direct current motor [3 which is suitably connected to the electrode. As shown, the shaft M of the motor is connected preferably through suitable speed reducing gearing to a sheave l5 over which passes a cable is connected to the electrode, the other end of the cable being provided with a counterweight l1.

Direct current is supplied to the motor I3 by means of a special high amplification generator I 8 of the armature reaction type. This generator has a set of output brushes l9 and which are connected to the terminals of the motor l3 and a second pair of brushes 2| and ZZ'WhiCh are permanently short-circuited through a conductor 23 and which are in quadrature relation with the current supply brushes l9 and 20. Also, the generator I8 is provided with a series compensating winding 24 which substantially neutralizes the armature reaction of the generator along the current supply brushes l9 and20; together with a main field excitation winding 25 which provides magnetic excitation along the secondary axis of the brushes l9 and 21']. The sensitivity of control of the voltage by variation of the excitation of the winding 25 is increased by means of the field winding 24 which supplies excitation in the same direction as the field 25 but proportional to the current supplied to the motor l3. With this arrangement the control field winding 25 is required to supply only a relatively small excitation and, consequently, it has both a low resistance and a low inductance which provide for very rapid changes of current in it.

The field winding 25 is connected for energization across two voltages in opposition to each other so as to be energized in response to the difference in these voltages. One of these voltages is derived from a current transformer 26 connected in the electric supply circuit for the vmeans 29 and 30 respectively. Also, to provide for adjustment, the voltage of the rectifier an 'is applied to a resistance 3| which, together with the rectifier 29, is connected in circuit with the field winding 25.

7 From its left-hand terminal the circuit of the coil 25 leads through a normally closed switch 32forming a part of an individual electrode controller 33, a normally closed switch; 34 controlled by a master controller 35, the-rectifier 29, an

adjusted portion of the resistance 3 I, an adjusted portion of a regulating resistance 36, a normally closed switch3'l operated with the switch 34, a normally closed switch 38 operated by the controller 33 and conductor 39 through the normally closed raise and lower limit switches 40 and 4|, and conductor 42 back to the right-hand side of the winding 25. I p I When the electrode I is raised out of contact with the charge II and no'arc is'established; as in the starting of the furnace, no current, of

course, flows in the electrode and consequently a the entire voltag selected from the resistance 3|. is applied to the generator field winding 25. This voltage energizes theifield .winding in a direction tocause the motor to lower the. electrode. This initial lowering operation, however, is preferably ata relatively slow speed,: and .to effect this the resistance 36. is provided which is adjusted to give thedesired. slow loweringspeed for striking the arc. v

When the electrode is lowered far enough to come in contact with the charge, the primary winding 28 is thus short-icirc'uited and the voltage across the rectifier:3llfis reduced to zero. ever, a large surge of current occursnat. this, time in the. electrode circuit in response to which a voltage. is impressed 0n the rectifier 29 for reversal of the excitationoofthe winding, and reversal. of the..electrode .rnotor.

series with a currentlimiting resistance 44 is energized and closes the switch 45 thereby shortcircuiting the resistancel36 for maximum energization of the field winding ".25 for quick raising or recovery of the. electrode to an arcing position, 7 and for high-speed adjustment of the'electrcde during the operation .ofjthelfurnace. The electrode is raised.untiltheielectrode. current is of l such value that the .voltageoutput' of the rectifier 29 is substantially...equa1,tothe, voltage from the resistance '31 in serieswith it. i'Under. these conditions, since .the- .two voltages oppose each other, no current flowsin the. generator field and the motor is consequentlydeenergized so that the electrode is held in thatiposition.

The relay switch 45remains'closeda's, long as current flows inthe'electrdjde circuit. "If for any reason the arc isextinguished, the switch 45 immediatelyopensf and the electrode is lowered at a slow speed againto reestablishthe arc.

The position of'the electrodecorresponding to thesettingof the resistance}! is now held automatically. ff If the furnace current, decreases; the

' voltage from the .rectifien29."likewise'decreases' and the field winding '25,.is'energized in a direction to lower the'electr'ode until balance is again restored. Conversely, i'fIthe' furnacecurrentin- V lowering limit switch 4 I This .re'ctifi'er passes current only in a. directionto raisethe electrode and provides for theraising of'theelectrode in the event thattheelectrode'is driven to. its lower limit and opens the limit switch '4[ Ordinarily How- Immediately the coil 43 connected acrossthe rectifier 29 in the electrode will not be raised to its upper limit and open the limit switch 40 but if this should occur the control must be transferred to manual 'operation as hereinafter describedfln order to lower the electrode.

For the purpose of increasing the inherent anti-hunt characteristics of the apparatus, an

additional field winding 48 is provided for the generator, this, fieldwinding being connected di- -rectly across.the armature of the generator. Its polarity'is'such that it always opposes the main field .winding 25 with the result that it gives an anticipatory action. The strength of the antihunt field from the winding 43, and hence sensitivityof the generator, may be varied by varying ,thejstrength of energization of the winding 48 lby. means, of a variable resistor 49in series with of the resistance49'when the electrode is being thewindin 4a.

A' blocking rectifier 5B i provided across asection of the resistance 49 in a direction to pass'current and short out a portion lowered. This provides a separate'adjustment to reduce the lowering speed, to a much lower value than the raising speed'thereby to prevent breakage of the electrode, such as when melting heavy scrap metal.

An iron core saturable reactor-5| is connected across the current transformer 26. This reactor saturates in the event of heavy current surges, such as are commonly encounteredat the beginning of the furnace operation-when the arc is made andebro-ken rapidly before the furnace current reaches asteadystate. The saturation of the rea-ctor 'Ellimits to a safe valuethe-voltage that can be impressed on the rectifier 29.

' Itwill be'understood-that a regulator for the three-phase furnace shown'has for each phase duplicate regulating equipment as thusfar described.

Forthe manual control of the furnace the in-' dividualcontroller -33-and the master controller =35 are provided, there being an individual controller '33 for each electrode and a single master controller 35 for all two or three electrodes of the furnace. During automatic operation the handle 52 of themaster controller is pulledout whereby its switch 53 is closed and the handle isthereby locked against turning so that its switches 54 and 55 arelocked open. -The switch 53 energizes a coil-56 which maintains closed the switches 34 and 31 and also closes a third-switch 51 whose function is-to makeeffective theeindividual controller 33.

Also; similar; switches '-(not shown) actuated-by coils in parallel-with the coil-56 are closed for the controls-on each of-the -other electrodes of the furnace. 7

Also during automatic operation} the handle 58 of --the individual controller -33-ispulled out and-thereby locked-against turning wherebythe switches '22 and -38 are closed-fo r automatic operation;

,"If it isdesired now to -control one electrode manually, leaving-the other electrodes on automatic operationgthis may be done with the in- 'dividu'al controllerfi3-3 ior that electrode. The handle'fifi is pushecldnwhereby the switches 32 and SS-are openedandthefield winding-=2 5'- thus disconnected; from theyautomatic control appa- 'ra'tus. Current for energizationof the field'winding 25 is now supplied from a'resistance 59connected across direct current supply lines fiii and ti. 'Alsowhen the handlef-58 is pushed in, the

switch Ha lS 1OSBd and "remains-closed when "the handle is turned. 'i Turning-the- -switch 33 clockwise to a --raise"- posi ti-oncloses-theswi-tch three different voltages.

2,351,420 ti, and also a switch 63, whereby the field winding is'connected across the points 64 and 65 of the resistance 59 and is energized in a direction to raise the electrode. This circuit leads from theleft-hand side of the field through the switch 51 and switch 62, both of which are closed, to

the point 64, thence through theresistance to the point 65 and through the switch 6la, conductor 39, limit switch 48, conductor 65a, switch 63, and conductor 42 back to the right-hand side of the field winding. It will be noted that the switch 63 by-passes the lowering limit switch 4|. To lower the electrode, the controller 33 is 1 turned counterclockwise. to a lower position whereby theswitches 66 and 61 are closed, and the switch 61a will be likewise closed. This connects the field winding across the. points 65 and 68 on the resistance, the circuit leading from the left-hand side of the field winding through the switch 51, the switch 61, to the point 68, through the resistance to point 65, switch 6Ia, conductor 39, switch 66, conductor 65a, limit switch 4 I, and conductor 42 back to the right-hand side of the field winding.

In the event that it is desired to raise or lower all of the electrodes manually as a group, the master switch 35 only is used. The handles 58 of the individual controllers may at this time be in either their automatic or manual operating positions.

The handle 52 of the master controller is pushed in whereby the switch 53 is opened and the field winding of each motor isolated from its control by the opening of the corresponding switches 34 and 31 and the switch 51. Turning the handle 52 clockwise to the raise position closes the switch 54 whereby the raise coil- 69 is energized, together with similar parallel connected raise coils (not shownrfor theother electrodes. This coil 69 closes the switches 10 and H whereby each field' -winding is connected across the points 65 and 112 of resistance 58 for energization in a raise direction.

To lower the electrodes, the master switch is turned counterclockwise to the lower position I thev field winding being used as described during automatic control. Thus the field excitation means may consist of one or more field windings.

Ordinarily in the operation of the furnace,

several voltages are used as desired in the melting of a charge. As shown, the primary winding 11 of the transformer supplying energy to the electrode I8 is provided with three taps which may be selectedby a rotatable arm 18 to give When the voltages change, however, it is necessary to adjust the value of the voltage applied by the secondary winding 21 to the rectifier 38 so that the voltage actually applied to the rectifier remains unchanged. This is done by correspondingly changing taps on a regulating resistance 19 in circuit with the rectifier. Thus we have shown a short-circuiting switch arm 80 mechanically connected by a link 8| to the transformer switch arm 18 so that a change in transformer voltage makes simultaneously a corresponding oif setting change in the resistor 19 and the voltage applied to the rectifier remains unchanged.

An advantage in using the arc voltage as a bias voltage or as a standard value against which the current is compared is obtained in the control of the furnace under conditions causing very low arc voltage. For example, when a large piece of metal falls against the electrode andshort-circuits the arc, the current in the electrode circuit rises to a very high value and the arc voltage drops to practically zero. Under these conditions, the greatly increased current energizes the field winding 25 so as to cause raising of the electrode and, in addition, this effect is still further increased by the greatly increased differential control voltage caused by the drop in arc voltage. In the case of a threephase furnace, this short-circuiting of one are causes an increase in the current in the other two electrodes, but with an increase in their are voltages so that the increase in current is substantially offset by the increases in arc voltages, and very little regulation of these two electrodes takes place.

It will be understood that any suitable construction can be used for the manually operated switches 33 and 35. As shown diagrammatically, the switch 33 is provided with an operating shaft 82 on which is splined a shaft 83 carrying the movable contacts of the switches 62, 63, 66 and 61. Thus when the handle is in its pushed in or lowered position, as seen in the drawing, it may be turned from the mid position shown in a clockwise direction looking down on the handle to close the switches 62 and 63, or turned in a counterclockwise direction from this mid position to close the switches 66 and 6?. The shaft 82 carries a disc 84 having a notch 85 which registers with a stationary stop 86 when the shaft is in the mid position shown so that the shaft can be pulled out or upward, the sides of the notch embracing the stop 86, thereby to close the switches 32 and 3B. When the shaft is thus pulled out, the stop 86 prevents turning of the shaft and thereby prevents closure of the switches 62, 63 or 66, 61. Also carried by the shaft is a second disc 8'! which when the shaft is pushed in or downward, as shown, closes and holds closed for all positions of the shaft the switch 6 I a. This switch is biased open as by a suitable spring and opens when the shaft and disc 31 are moved upward.

The master switch 52 is shown diagrammatically in a similar manner, this switch having an outer sleeve 88 splined on its shaft and operating the switches 54 and 55. It also is provided with a look (not shown) similar to 84, 85 and 86 of the switch 33 assuring that the switches 54 and 55 are open when the switch is raised.

While We have shown a particular embodiment of our invention, it will be understood, of course, that we do not wish to be limited thereto since many modifications may be made, and we therefore contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of our invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. A control system for a furnace provided with an electrode comprising a transformer for supplying current to the electrode, a motor for moving the electrode thereby to vary the current in the electrode and the voltage between the electrode and a charge, a generator for supplying current to said motor tooperate said motor,

field excitation means for said generator, means differentially responsive to the current in, the

and a charge for energizing said field excitation means thereby to cause said 'moto'rto movethe electrode and maintain a predetermined current in the electrode, means for varying the voltage I supplied by said transformer to the'electrode,

means for varying the response of said differential means to said voltage, and meansifo'rsi- Imultaneously actuating said two varying means to maintain a predetermined response of said difierential means to said Voltage independently" of the value of said voltage.

2. A control system for a furnace provided with an electrode comprising a motor for moving the electrode thereby to vary the current in ,the electrode and the voltage between the elec-"" trode and a charge, a generator for supplying current to said motor to operate said motor, field excitation means for said generator; means differentially responsive to the current in the elecelectrode and the voltage between the electrode ,4 agnga voltage between the electrode and a charge for "energizing said field windingthereby to cause said motor to raise or lower said elect-rode and m'airltain' a predetermined current in said electrode, an anti-hunt field-winding for said generator "connectedto said generator, a resistance incircuit with said field-Winding, and a blocking rectifier connected across at least a portion of-said resistance soas toflshort-circuit" said portion 'of said resistance when the elect'rode is being lowf dthe'reb'y to reduce the electrode lowering s eed.

5i A control system for an arc furnace comprising a transformer for 'supplyin'g'current to an electrode; a motor for raising and lowering the electrode thereby to-varythe'current insaid electrode and the voltage-between the electrode and a'chargaa generatorfor supplying current-to trode and the voltage between the electrode and a charge for energizing said field excitation means thereby to cause said motor tofrnove the electrode and maintain a predetermined current in the electrode, and means responsive-to the V surge of current in the electrode when the electrode is moved into engagement with the charge for increasing the energization of said field excitation means for increased speed of movement of the electrode during subsequent adjustment of the electrode to maintain said predetermined current.

3. A control system for an arc furnace comprising a motor for raising andloweringi'the' electrode thereby to vary the currentin the electrode and the voltage between the electrode and a charge, current responsive means for producing .affirst voltage responsive to the current in the electrode, voltage responsive means for supplying a second voltage responsive to the voltage between the electrode and a charge, differential-- voltage supply means for rectifying and compar- 1 ing said voltages and for supplying a third voltage responsive to the difie'rence between said first and second voltages, a generator for supplying current to said motor to operate saidmotor, field excitation means for said generator, a resistance,

, connections for connecting said differential voltage supply means to said field excitation means in series with said resistance thereby to "cause said motor to raise and lower said electrodein' dependence upon the polarity of said third voltage and maintain a predetermined current said electrode, and means responsivefto 'said first voltage for shortecircuiting said resistance when the electrode touches the chargethereby to' increase the energization of said' fi'eld excitation meanscandthe speed of operation of said elecft rode 4. A control system for a furnace provided with an electrode comprisingsupply'means for supplying current toan electrode, a motor for'raising andlewering' theelectrode thereby tovary the current in the electrodejand the voltagebetween the electrode and a cha e, a;

said motor to operate saidmotor, field excitation means for said generator rneans difi erentially responsive to the current in saidi electrode and the V rrnature reaction type generator for supporting current to "said motor toopera'te said motona fieldfor said generator, means difierentially responsive to the current in said'electrode and the voltage between the electrode and-a charge for energizing said 1 field winding thereby to cause'said motor to raise or lower said electrode and 'maintain a predetermined current in said electrode, and meansfor varying concurrently thevoltage supplied by ing and lowering each-electrode thereby to vary the current in said electrode and the voltage between the electrode and a charge, current responsive means associated "with each electrode for producing afirst alternating voltage responsive to'the'current in the electrode, 'voltage're- 'sponsivemean's for each electrode for supplying a second alternating voltage responsive'to the voltage between the electrode and the charge, difierentialfvoltage supply means for each electrode for rectifying and comparing said voltages and for supplying a direct voltage responsive to the difference between said first andsecond'voltages; a "generator, for supplying-current to each of said motors, field excitation meansfor each of "said generators, connections for supplying aconstant directvoltage for said field excitation means, a manually operated control device for each of saidfelectrodes movable "to one position to 'connect'the field excitation means of the corresponding generator'to said differentialvoltage f "supply means, and movable toother'positions to connectsaid field excitation means'tosaid direct voltage supply connections so as to control the direction of energization of said field excitation 7 *means for manual control fof the corresponding '*electrode, and a second manually operated control idevice movable to one position to establish c'on'nectionsjfor each of said"fie1d excitation means with its 'difierential voltage supply means and'movableto other positions to connecteach of s'aidfield excitation'means to said direct voltge supply connectionsso asto control thedi- 1 (action of energizationof allof an field excitati-on means "for concurrent manual "control of all of said electrodes.

' l DONALD E. ean.

G QRG E-"TSHAA 

